Seismic waves that vibrate perpendicular to the direction of their path

Answer: (a) Travel through the solid parts of the earth only

S waves (transverse) travel only through solids — they can’t pass the liquid outer core.

The idea

The perpendicular particle motion described here identifies a shear body wave. Shear needs a material that resists a change of shape; a liquid cannot provide that resistance. This is why the wave is useful for identifying solid and liquid regions inside Earth.

A shear disturbance enters solid rock → Particles move across the travel direction → Elastic resistance passes the disturbance onward → Wave reaches a liquid boundary → Shear cannot pass directly through the liquid

Why the others are wrong

(b) That describes P waves.

(c) That describes surface waves.

Elimination technique

Read perpendicular motion as the S-wave description used in this question. A liquid cannot transmit a shear body wave, excluding the solid-plus-other-media option; surface travel describes another wave category.

Also know

P waves compress material and can cross solids and liquids. The liquid outer core blocks direct S waves, but waves can convert between types at boundaries. The mantle is mostly solid even though it flows slowly over geological time. The statement concerns S body waves; surface waves have their own particle-motion patterns.